Few products are sold by their manufacturer without some type of testing being conducted. Such testing may be as simple as manually ascertaining whether certain parts are securely affixed—or as complex as “stress testing.” In stress testing (or “stress screening” as it is sometimes called), products exhibiting “infant mortality” fail outright during the test. Or as the result of such testing, a product may display evidence of early failure in the operating environment.
One of the most common methods of stress testing involves testing a product by subjecting it to vibrations of the type which might be encountered in actual product use. For example, U.S. Pat. No. 2,438,756 (Larsen) explains that the apparatus described therein is used to vibration-test electrical apparatus for airplanes, ships and the like. The unit described in U.S. Pat. No. 3,748,896 (Barrows) is said to be used for testing parts of a motor vehicle. And vibration testing is often conducted in conjunction with testing using another regimen, e.g., temperature.
One type of vibration testing is known as repetitive shock testing. Such testing is generally accomplished by utilizing a testing apparatus consisting of a table frame that is vibrated by a number of vibrators. These vibrators are generally pneumatically powered. During the testing process a uniform vibration response is desirable because it ensures that all components being tested are exposed to approximately equal vibration levels over the entire table frame. This uniform response is created through the use of those multiple vibrators. However, if one or more of the vibrators have reduced performance, the vibration response becomes non-uniform. Attempting to compensate for this non-uniform testing can create a magnification of stress to certain components in the assembly while minimizing stress to others, potentially yielding a false representation of strengths and weakness in the assembly.
Systems have been developed to monitor vibrators for failure characteristics prior to the point of failure. For example, the V-flare system was developed by a company called Data Flare (no longer in business); this system worked by attaching sensors directly to each vibrator and monitoring the strength of the impacts. This system is an add-on to known systems and introduces extra parts that may fail and add higher cost into the testing procedure.